The invention is based on a priority application EP 02 360 029.9 which is hereby incorporated by reference.
The present invention relates to an optical waveguide phase shifter comprising a planar silicon-dioxide-containing optical glass waveguide whose core contains glass, a planar optical polymer waveguide whose core contains a polymer and/or a mixture of polymers and also means that effect the change in temperature of the planar optical polymer waveguide. In addition, the invention relates to a method of changing the phase in optical waveguides.
Waveguide phase shifters or waveguide phase switches have already been described in the prior art. Thus, for example, it is known from EP 0997765 A2 to use optical waveguide switches that comprise a plurality of a number of glass waveguides disposed in parallel in a first layer, at least one polymer waveguide that encloses an acute angle with the glass waveguides being disposed in a layer situated on top thereof. Wherever the polymer waveguide crosses the glass waveguide situated thereunder, vertical coupling regions are produced. With the aid of heating electrodes, the coupling characteristics can be influenced in a selective manner. Consequently, light can be switched over from one glass waveguide into another glass waveguide disposed in parallel thereto, the temperature of the vertical coupling regions being adjusted in such a way that light coupled from the one glass waveguide into the polymer waveguide is relayed therein and coupled down in another coupling region into the desired glass waveguide.
Furthermore, the publication by N. Keil et al. entitled xe2x80x9cHybrid Polymer/Silica Vertical Coupler Switchesxe2x80x9d in EC10 2001 discloses optical waveguides for optical fibre communication networks that are used, for example for switching or for optical routing. In that case, a vertical coupling likewise takes place from silicon dioxide waveguides and polymer waveguides that are disposed above one another and wherein the temperature of the polymer waveguide can be controlled by means of heating elements. In the abovementioned article, a vertical coupling of the light conducted in the glass waveguide can be achieved by suitable choice of carrier layer, of the silicon-dioxide-containing waveguide layer and also of the polymer waveguide layer. However, the silicon-dioxide-containing waveguide layer and the polymer waveguide layer are in that case separated from one another by a so-called polymer xe2x80x9cgap layerxe2x80x9d. In this case too, the light is conducted during or after the coupling in the polymer waveguide.
The demand for ever higher data rates for transmission in optical communication networks necessitates an increasing need for the correction of the transit-time errors on the (optical fibre) transmission link. The correction also consequently becomes necessary of higher-order chromatic dispersion (CD) or, alternatively, of polarization-mode dispersion (PMD). Fast optical phase shifters are necessary for the development of optical PMD or CD compensators.
The prior-art concepts discussed above and likewise usable for the abovementioned purpose do not satisfy, in particular, the requirements for speed, energy to be applied and any desired modification of the optical phase.
The object of the present invention was therefore to provide an optical waveguide phase shifter that makes it possible to modify the phase of optical signals as desired, wherein only a small energy is necessary and wherein the optical waveguide phase shifter can, in addition, easily be extended to a higher number of transmission channels.
This object of the present invention is achieved in that the optical waveguide phase shifter according to the invention comprises a planar silicon-diode-containing optical waveguide whose core contains glass, and also a planar optical polymer waveguide whose core contains a polymer and/or a mixture of polymers, furthermore means that effect the change in temperature of the planar optical polymer waveguide, and wherein the refractive index of the planar optical polymer waveguide is greater than the refractive index of the planar silicon-dioxide-containing optical waveguide.
Since the light conduction in the waveguide phase shifter disclosed here takes place only in the glass, the polymer waveguide used can be of very thin construction. This achieves the result that the refractive indices of the planar optical polymer waveguide (referred to below for the sake of simplicity as xe2x80x9cpolymer waveguidexe2x80x9d) and of the silicon-dioxide-containing optical glass waveguide (referred to below for the sake of simplicity as xe2x80x9cglass waveguidexe2x80x9d) are matched even by a small increase in the temperature of the planar optical polymer waveguide and only the propagation constant of the optical field in the silicon-dioxide-containing optical waveguides is influenced by the coupling mode between glass waveguide and polymer waveguide.
The waveguide phase shifter according to the invention is consequently based on a vertical glass/polymer hybrid waveguide switch, the polymer waveguide being designed however in such a way that the optical wave cannot be propagated in it.
Light is not therefore coupled vertically into the polymer waveguide, with the result that only the phase of the optical field in the glass waveguide is influenced. In contrast to the abovementioned achievements of the prior art, the light is not conducted in the polymer waveguide, despite the coupling. This has the result that the energy to be applied to increase the temperature is very small (typically, the electrical heating power needed is  less than 20 mW) and is also rapidly operative.
Surprisingly, all these advantageous effects have the result that the modification of the phase is possible with a very short response time of approximately 0.1-3, preferably 0.1-2 milliseconds.
This short response time is a basic prerequisite for the desired application in CD- or PMD-compensation components.
It is preferable that the polymer and/or the mixture of polymers of the planar optical polymer waveguide have/has a pronounced thermooptical effect. The refractive index of the polymer waveguide can thereby easily be matched to the refractive index of the glass waveguide by the selective temperature change, only a small energy being necessary, as a result of the pronounced thermooptical effect, to heat the means that effect the change in the temperature of the polymer waveguide.
In a preferred embodiment, the core of the planar glass waveguide is surrounded by a waveguide cladding layer that brings about a change in the refractive index of the glass waveguide. Consequently, the refractive index of the core of the glass waveguide can easily be adjusted and varied in a selective manner.
Preferably, two waveguide cladding layers are provided, of which the one comprises a polymer and/or a mixture of polymers and the other comprises silicon dioxide, with the result that the refractive index can be adjusted particularly simply and well by a systematic choice of the thickness and sequence of the respective waveguide cladding layers.
It is furthermore preferable that the core of the polymer waveguide is constructed in strip form. Because of its small and systematically adjustable thickness, the temperature gradient is consequently particularly small in its interior, with the result that the temperature changes due to the heating elements can be transmitted to the polymer waveguide without high temperature and time losses.
In an advantageous embodiment, the thickness of the core of the glass waveguide is 4-10 xcexcm, preferably 5-8 xcexcm. Furthermore, it is preferable that the thickness of the core of the planar polymer waveguide is 0.5-5 xcexcm, preferably 1-3 xcexcm.
The thickness of the glass waveguide and polymer waveguide and the thickness of the interlayers are adapted in accordance with the requirements relating to the maximum value of the phase shift and the wavelength of the light. The adjustment of the refractive index and, consequently, also the matching of the refractive indices of the polymer waveguide and of the glass waveguide are also influenced thereby.
Preferably, the means that effect the change in the temperature of the planar optical polymer waveguide are designed as heating elements, well known per se to the person skilled in the art, that can be easily provided and also easily heated.
The object of the present invention is furthermore achieved by a phase-change method in optical waveguides, wherein said method is performed using a waveguide phase shifter according to the invention, wherein the temperature of the planar optical polymer waveguide is changed in such a way that its refractive index is reduced with the result that light is coupled from the glass waveguide only into the polymer waveguide.
The method according to the invention achieves, in a simple way, the result that the propagation constants of the glass waveguide and polymer waveguide are matched and consequently only the phase of the light still conducted in the glass waveguide is altered. Consequently, the phase shifter operates without loss and, in particular, the material losses of the polymer waveguide for this application are insignificant.
Preferably, the use of an optical waveguide phase shifter according to the invention or of the method according to the invention in optical data transmission devices and in their individual components, for example in Mach-Zehnder interferometers, polarization-mode equalizers, dispersion equalizers, gain equalizers or Bragg gratings.
Definitions of all the abovementioned devices and components are to be found in xe2x80x9cGlossary of Telecommunication Termsxe2x80x9d FED-STD-1037C of the US xe2x80x9cGeneral Services Administrationxe2x80x9d Authority, to which reference is comprehensively made here.
Further advantageous refinements of the present invention are explained in the drawing below and by reference to an exemplary embodiment.
It goes without saying that the advantages and features of the invention mentioned above and explained below can be used not only in the specifically described combination, but also in other combinations accessible to the person skilled in the art without further thought or in isolation without departing from the scope of the invention.